CONSIDERATIONS WHEN TRAINING WITH FOOD:
Most puppy training starts with food training. Puppies are maturing, and do not have the focus of an adult dog. They are discovering the world with their paws, nose, eyes, and mouth. Therefore, humans need a good reward method to catch their attention for any period of time.
Adult dogs will have more maturity and focus (usually). Therefore, the first section does not necessarily apply to them. Although, these are things you may want to consider, especially if your adult dog seems at all flat when training. Both young and old dogs need a distraction free place to start learning at first.